1. Field of the Invention
The present invention relates to a pseudo-Schottky diode that has an n-channel trench MOSFET.
2. Description of the Related Art
In three-phase or alternating-current generators for motor vehicles (alternators), alternating-current bridges (rectifiers) are used for power rectification. Usually semiconductor diodes having a p-n junction made of silicon are used as rectifying elements. In a three-phase generator, for example, six semiconductor diodes are interconnected to form a B6 bridge. Occasionally, diodes are also connected in parallel and twelve instead of six diodes are used, for example. In alternating-current generators having a different phase number, suitably adapted diode bridges are used.
The diodes are designed for operation at high currents or current densities of up to more than 500 A/cm2 and high temperatures or a maximum barrier layer temperature Tj of approximately 225° C. At the high currents used, the voltage drop in the forward direction, i.e. the forward voltage UF, is typically approximately 1 volt. In operation in the blocking direction below breakdown voltage UZ, normally only a very small reverse current IR flows. From breakdown voltage UZ onward, the reverse current rises sharply. A further voltage rise is therefore prevented. In this context, Z diodes are used having reverse voltages of approximately 20-40 volts, depending on the system voltage of the respective motor vehicle. In the breakdown, Z diodes may be loaded briefly with high currents. For this reason, they are used to limit the overshooting generator voltage during load changes. Such diodes are usually packaged in robust press-fit diode housings, as is described for example in published German patent document DE 195 49 202 B4.
The forward voltage of p-n diodes results in forward power losses and thus in an efficiency degradation of the generator. Since in the current delivery of the generator on average two diodes are always connected in series, the average forward power losses in a 100 A generator amount to approximately 200 W. These losses result in a temperature rise in the diodes. The arising heat must be dissipated by elaborate cooling measures on the rectifier, for example by using heat sinks and/or a fan.
To reduce the forward power losses, published German patent application document DE 10 2004 056 663 A1 provides for the use of so-called high-efficiency Schottky diodes (HED) instead of p-n diodes. High-efficiency Schottky diodes are diodes in which, in contrast to conventional Schottky diodes, the reverse current is nearly independent of the reverse voltage. High-efficiency Schottky diodes are made up of a combination of a conventional Schottky diode (SBD) and additional elements such as magnetoresistors, p-n junctions or different barrier metals, the combination being monolithically integrated on a semiconductor chip. High-efficiency Schottky diodes are frequently implemented in trench technology. In addition to the low forward voltage in the forward state, they also limit the overshooting generator voltage, which may occur in sudden load changes, to non-critical values, in 14 V systems typically to voltages below 30 V.
Using high-efficiency Schottky diodes, it is possible to realize substantially lower forward voltages UF in the range of 0.5 V to 0.7 V. The low forward power losses of the diodes increase the efficiency and the power output of the generator. As a consequence of the lower blocking-state power losses, it is additionally possible to reduce significantly the expenditure for cooling as compared to the use of p-n diodes.
A manufacture of high-efficiency Schottky diodes is expensive and technologically very demanding. In addition to very fine trench structures with measured widths in the range below 500 nm, which must be etched into the silicon, particularly the cost-effective manufacture of suitable and stable Schottky contacts represents a challenge. Nickel silicides or other suitable silicides are preferably used as Schottky contacts. In modern semiconductor manufacturing plants, which produce performance MOSFETs, these silicide processes are usually not available.
Published German patent application document DE 10 2010 062 677 A1 provides for the use of so-called pseudo-Schottky diodes (PSD) instead of high-efficiency Schottky diodes. These are specially manufactured n-channel MOSFETS having an extremely low threshold voltage Vth, in which gate, body and source regions are electrically fixedly connected to one another and act as an anode, while the drain region functions as a cathode. For the use of pseudo-Schottky diodes, like the use of high-efficiency Schottky diodes, allows for the implementation of low forward voltages and a voltage limitation. The voltage limitation in this instance occurs with the aid of the built-in body diode. Such components are known, for example, from U.S. Pat. No. 5,818,084. These semiconductor components contain no Schottky contacts and therefore require no special silicide processes. They may be produced using modified standard processes for MOSFETs. As majority carrier components, they in turn switch very quickly.
Pseudo-Schottky diodes are manufactured in planar power semiconductor MOSFET technology. The avalanche breakdown occurs at a voltage UZ in the interior of the semiconductor on the so-called body diode D. In the avalanche breakdown or reverse voltage breakdown, hot charge carriers (electrons and holes) are generated. The electrons flow to the cathode while the holes flow directly via the body region to the anode. The special construction of the planar MOSFETs ensures that no hot holes are injected into the gate oxide in the process, but that rather they flow off directly via the body region. The component may be operated during a load dump in the breakdown and thus limit a voltage rise without its gate oxide being modified or without the component being damaged.
In pseudo-Schottky diodes having the considered reverse voltage range, the voltage in the forward direction, the forward voltage UON, essentially drops off in the channel region (inversion channel). In order to achieve forward voltages that are as low as possible, the channel width or the number of channels in relation to the utilized chip area should therefore be selected to be as great as possible.
Fundamentally, the cell density and thus the channel width may be increased markedly if power MOSFETs, which are produced in trench technology (trench MOSFETs), are used instead of planar power MOSFETs. In these components, however, the avalanche breakdown occurs preferably on the bottom of the trench structure. The disadvantage in this regard is that the holes that are produced in the avalanche breakdown are accelerated by the electric field and are injected into the gate oxide. There they act like positive charges fixedly built into the gate oxide. This changes the properties of the trench MOS field-effect transistors. The threshold voltage VTH is lowered, for example, while the reverse voltage is increased. The variation has particularly disadvantageous effects in trench MOS transistors that have a very low threshold voltage. For this reason, pseudo-Schottky diodes, which are based on trench MOS concepts, are not suitable for voltage limitation.
Another disadvantage both in planar as well as in trench concepts is the known fact that the hole current generated by the avalanche effect generates a voltage drop on its way to the source contact and may thereby turn on the npn transistor formed by the source region, body region and n epilayer. The result is an abrupt reset of the breakdown voltage (snap back), which may result in a destruction of the transistor. This effect is particularly pronounced at high temperatures and high breakdown currents.